Ionic elastomers such as solfonated ethylene-propylene-ethylidene norbornene terpolymer (Sulfo-EPT) have been developed recently. See U.S. Pat. No. 3,642,728. These materials under carefully controlled conditions can be formulated or compounded to produce products which can be processed at elevated temperature as thermoplastics, yet at ambient temperature, possess the rubbery character of vulcanized elastomers. These systems are similar to a new class of materials, designated as thermoelastomers.
While the properties of these materials, such as Sulfo-EPT, are similar to those of vulcanized elastomers in many respects, they are markedly deficient in one important area, i.e. compression set. Compression set, as described under ASTM test conditions, (ASTM D-395), is a measure of an elastomeric material's ability to recover its original shape, after being subjected to a severe compressive load (25% reduction in thickness) for extended time periods (usually 22 hours). While vulcanized elastomers often possess compression set values for room temperature compression of 20 to 25%, and values of 30% or so at compressive load temperatures of 70.degree.C., the values obtained for Sulfo-EPT are much worse. For example, it is common to observe values of compression set at room temperature for Sulfo-EPT based materials on the order of 50 to 80%. At a temperature of 70.degree.C., the compression set values are substantially worse, being on the order of 95-100%. In practical terms this means that such Sulfo-EPT compositions are severely limited in their potential applications at temperatures which are near 70.degree.C. or above. In effect, at such temperatures it appears that Sulfo-EPT based systems undergo a certain amount of irrecoverable flow when placed under compressive load. Such a deficiency signifies that Sulfo-EPT based materials may not compete in those applications wherein compression set is important such as gaskets, automotive coolant hose, and many sealant uses.
Therefore, it is clear that it would be extremely desirable to improve this characteristic of Sulfo-EPT.
However, such an improvement should not be achieved at a major sacrifice in polymer fabricability. For example, it has been shown that Sulfo-EPT when prepared with a suitable sulfonate level, and with a suitable cation can possess good compression set properties. However, when attempts are made to process such materials even at very high temperatures, it becomes evident that these materials are not processable under practical conditions such as extrusion, injection molding, or even compression molding. Therefore, the combination of good compression set (i.e., values below 80% at 70.degree.C.) and ease of fabrication (i.e., processability) has never been observed with a material based on Sulfo-EPT.